'unappeasability' originates from Modern English, specifically formed from the negative prefix 'un-' (from Old English 'un-') + the verb 'appease' + the nominal suffix '-ability'/'-ity', where 'un-' meant 'not' and 'appease' meant 'to pacify'.

'appease' comes from Old French 'apaisier' (to pacify), ultimately from Latin roots related to 'pacificare'; Middle English and Early Modern English produced forms such as 'appease' and 'appeasable', later yielding the negative adjective 'unappeasable' and the noun-forming 'unappeasability' in modern English.

Initially based on the sense 'not able to be pacified' (from 'appease'), the term has come to be used more broadly to describe an extreme unwillingness or inability to be satisfied or placated in many contexts (personal, political, etc.).